Katarina Johnson-Thompson secured gold at the world junior championships with a third-round jump of 6.81m. Photograph: David Ramos/Getty Images

The Liverpool teenager Katarina Johnson-Thompson gave an emphatic boost to her Olympic preparations by winning long jump gold at the world junior championships in Barcelona on Friday.

Johnson-Thompson's third jump of 6.81m was enough to finish top of the standings although she had an agonising wait to claim victory as Germany's Lena Malkus reached 6.80m on her final leap.

It capped an impressive day for Johnson-Thompson, who has qualified to represent Great Britain in the heptathlon at the London Games, having earlier clocked a personal best 13.48sec to safely qualify from her 100m heat.

Johnson-Thompson, whose winning jump was not classed as a personal best because of an illegal wind speed, said: "I am really pleased with that because I didn't expect it. I thought I would be in with a shot at the top five or the top three maybe, if I hit the board right.

"I always knew in my head there was a big jump coming. This time it all came together when it really mattered. When it comes to peaking at the right time, I have to thank my coach Mike Holmes – he is a genius.

"When the German athlete did her big jump I couldn't really see what distance it was – I had to ask an official. What a relief to find out it was 6.80m, not 6.82m – I don't think I could have responded as my big jump had taken all my adrenaline out of me."

Despite her success Johnson-Thompson – who has been tipped for a great future by both Jessica Ennis and Denise Lewis in recent weeks – insisted the heptathlon would always remain her main event.